1. Field of the Invention
The present invention relates to a picture taking tube with pyroelectric target. It also relates to a process for determining the axes of least expansion of the target.
2. Description of the Prior Art
Picture taking tubes with pyroelectric target are well known in the prior art. They are used for taking pictures in thermal television and in particular in the infra red field.
The present invention relates more particularly to the pyroelectric targets.
FIG. 1 is a sectional view showing a pyroelectric target 1 and a circuit which is used in the prior art for supporting this target, biasing it and reading it.
The pyroelectric target 1 is fixed on a thin plate 2 which is itself stretched over a thick ring 3.
This type of support protects the target, which is very fragile, as well as possible from the vibrations to which it may be subjected in the tube in which it is mounted.
An electrode 4, transparent to the radiation to be detected, is deposited on one of the faces of the pyroelectric target.
This electrode 4 is deposited on the face of the target which does not receive the electron beam, symbolized by a straight arrow in the Figure and which serves for reading the charge created on the target by the temperature variation.
An electrode 5 is similarly deposited on the face of the thin plate 2 which does not receive the radiation to be detected. This radiation is symbolized by a wavy arrow in FIG. 1.
Target 1, coated with electrode 4, on the one hand, and ring 3 and the thin plate 2 coated with electrode 5 are assembled together by means of a bonding layer 6.
The read-out signal from the target is collected on electrode 5 which also provides biasing of the target.
There is no connection between the two electrodes 4 and 5 but only a capacitive coupling.
The problem which arises is that the capacity which is seen by the electron beam serving for reading target 1 is formed by the resultant of the capacity of the target and of the bonding layer 6 mounted in series. The signal read by the team is multiplied by the ratio of these capacities C.sub.bond /C.sub.target and the sensitivity of the target is reduced.
To try to avoid this reduction of sensitivity, the two electrodes 4 and 5 have been connected directly together. Thus, the capacity due to the bonding layer 6 is removed. Moreover, an annular shaped electrode 5 may then be used which further increases the sensitivity, because the radiation to be detected does not have to pass through electrode 5 which is not perfectly transparent.
The applicant has however demonstrated that this solution was not satisfactory for the following reasons. The electrical contact formed between the two electrodes 4 and 5 produces a mechanical disturbance which very often causes breakage of the pyroelectric target during temperature variations. In fact, the thin plate 2 is formed from a resilient material such as a mylar (registered trademark) film; similarly, for layer 6 a bonding agent is chosen having resilient properties so as to allow expansion of the pyroelectric target 1. The introduction into this assembly of an electrical contact, hard from the mechanical point of view, tends to cause the breakage.
The present invention resolves in a simple and efficient way the problem which arises when it is desired to form a direct connection between the electrodes 4 and 5.